This is the default RAM address into which the kernel image is loaded (on the SheevaPlug) for execution.

Why this number? Because 0x0 - 0x800000 are reserved for U-Boot. (Really, look at the messages U-Boot prints when starting, before the kernel is copied.)

I've tested loading the kernel into other RAM addresses, and there does not seem to be a problem. But, if the memory manager is trying to handle two block (one above and one below the kernel), there might be a minor amount of extra work required. Generally, I'd suggest loading the kernel at the low end (0x800000) and allowing all the remaining memory to be managed as a single block.

With that in mind, it is very common to also see this address used in the commands fatload, ext2load, or tftpboot as a safe place to load an image into RAM temporarily before writing it to Flash.